Curtain wall is a term used to describe a building facade which does not carry any dead load from the building other than its own dead load. These loads are transferred to the main building structure through connections at floors or columns of the building. A curtain wall is designed to resist live loads forces: air and water infiltration, wind forces, seismic forces,—and its own dead load forces. The loads imposed on the curtain wall are transferred to the building structure through the anchors which attach the mullions to the building.
A mullion is a structural element which divides adjacent window units. A mullion may also vertically divide double doors. Mullions may be made of any material, but wood and aluminum arc most common. A mullion acts as a structural member, and it carries the dead load of the weight above the opening and the wind load acting on the window unit back to the building structure. Mullions are usually spliced to allow for expansion and contraction of a curtain wall and accommodate other building movements due to live loads, creep and displacement. Adjacent mullions are generally connected with a sleeve, which is usually made of the same material from which mullions are made. Usually, when connected mullions slide along a sleeve because of movements provoked by forces of live and dead loads, noise will occur clue to surface-to-surface friction and scratching between the sleeve's surfaces and mullions' surfaces. The resulted noise is then transferred through mullions to occupied areas of the building, and resulting in nuisance for structure's tenants. Further, a use of a bond breaker tape is generally required at a face of a sleeve to prevent three-sided adhesion when sealing the mullion joints. Any structure which serves to elevate the noise resulting from the movement of mullions along the sleeve connecting two adjacent mullions; would be of significant utility for construction industry.